Objects of type DOM_EXEC represent kernel functions implemented in C++.
Unlike functions defined at the library level (which are stored in objects of type DOM_PROC), functions defined in C++ in the MuPAD® kernel are represented by objects of type DOM_EXEC.
Most kernel functions are actually stored inside function environments of type DOM_FUNC_ENV, and therefore, you can see DOM_EXEC only when explicitly accessing the first or second operand of those function environments.
An object of type DOM_EXEC essentially represents a function; using it in this way calls the corresponding function.
The operands of a DOM_EXEC are used internally, may change at any time and remain undocumented.